Several techniques for cooperation between the entities of the system are known from the prior art. They depend on several parameters. These techniques depend, in particular, on the access technique used within the system. Thus:                by using a TDMA access technique (standing for “Time division multiple access”), the cooperation between a sender and a relay is effected in two steps. The first of these two steps is a phase during which the relay (and optionally the receiver) listens to the sender. The second step is a phase during which the relay processes the information that it has listened to and retransmits it to the receiver;        by using an FDMA access technique (standing for “Frequency Division Multiple Access”), cooperation is achieved by using two different frequency bands. The sender sends the information on a frequency band which is received by the relay (and optionally the receiver). The relay processes the information using a relay strategy and retransmits it using another frequency band. The receiver thus receives the information with a temporal shift on another frequency band;        by using an OFDMA technique (standing for “Orthogonal Frequency Division Multiple Access”), one and the same methodology as the FDMA case can be applied with the difference that the original signal of the source entity and that of the relay entity are transmitted on two different sub-carriers;        by using a CDMA technique (standing for “Code Division Multiple Access”), cooperation is achieved by using two spreading codes C1 and C2. The sender sends the information using the code C1. The relay processes the information and retransmits it using the code C2. The receiver thus receives the information from the relay with a temporal shift.        
Cooperation can also be used when other access techniques are implemented.
Cooperation techniques also use several relay strategies, as a function of the abovementioned access techniques implemented. Among these relay strategies may be cited:                amplification and then retransmission: the relay entity amplifies the signal received from the source entity and retransmits it to the destination entity;        decoding and then retransmission: the relay entity decodes the information from the source entity, it encodes it and then retransmits it to the destination entity;        quantization and compression followed by retransmission: the relay entity compresses the signal received from the source entity and then retransmits it to the destination entity;        hybrid schemes combining the above schemes.        
When the transmission channel established between one or more source entities and a destination entity exhibits poor radio conditions, the source entities may wait a lengthy period before being authorized to transmit by the scheduler. Moreover, when opportunistic scheduling is used (scheduling which favors the sources having the best radio conditions), those source entities for which the transmission channels established with the destination entity exhibit the worst radio conditions are rarely authorized to transmit.
Certain so-called real-time services impose a limit waiting time of the source entity before it is authorized to transmit not to be exceeded. This aspect has been solved in the prior art by way of an authorized maximum probability that the mean waiting time of a source entity exceeds a fixed threshold.
Prior art cooperation techniques have attempted to propose criteria for selecting the relays allowing a source entity to be chosen by a scheduler more frequently than it is without these techniques, in a mobile radio system supporting cooperation between its various entities (that is to say the various entities present in one of the cells of the cellular network).
This assumes the existence of signaling allowing the choosing of the relays allocated to a given source for a given destination as well as the start and end times of the phases of the cooperation when it is used. It is assumed that the transmission channels which link the source entity and the destination entity, and also the relay entities, are known by the scheduler.
When the radio conditions are bad, that is to say, when the transmission channel established between the source entity and a destination entity exhibits an attenuation such that the transmission of real-time services cannot be ensured, a source entity will wait for a long time before being selected by the scheduler, within the framework of opportunistic scheduling. In order to reduce this waiting lag, several prior art techniques exist:                priority allocation to the source entity and priority management at the level of the scheduler. The priority of the source entity is increased as a function of the waiting time. In this solution, the transmission channel established between the source entity and the destination entity remains unchanged.        compulsory transit through one or more fixed relay entities. Such a scheme consists in searching for a new route consisting of one or more fixed relay entities making it possible to improve the radio quality. This scheme has in particular been proposed for non-real-time services and for an uplink, that is to say a link from the source entity to a fixed destination entity such as a base station,        modification of the phase of the signal at the level of a relay entity. A signal is sent by the source entity destined for two distinct relay entities, each of these relay entities multiplying the signal received by a random phase. The combining on reception of the signals originating from each of the relay entities will give a time-varying global attenuation.        
Unfortunately, these earlier techniques exhibit a certain number of drawbacks.
Thus, when considering the technique of priority allocation to a source entity without changing the current transmission channel established with the destination entity, it is noted that this technique requires a change at the scheduler level. Moreover, such a technique does not guarantee sufficient transmission channel quality to be able to transmit. Finally, there is a risk that this priority allocation technique may considerably reduce multi-user diversity gain when opportunistic scheduling is used.
The technique of compulsory transit through one or more fixed relay entities requires for its part the existence of fixed relay entities in the cell. It therefore cannot be implemented in all typical cases.
Finally, the technique of modifying the phase of the signal at the level of a relay entity requires the presence of at least two relay entities. It is therefore expensive in terms of resources.